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commit d80222b97855fc3ffe9f7c1856cfd3740b5788dc
Author: Milos Nikic <[email protected]>
Date: Fri Jun 26 14:33:23 2026 -0700
ext2fs: Fix journal deadlock during transaction finalization
When a VFS thread finalizes a transaction in
journal_stop_transaction_locked, it holds the global j_state_lock while
performing memcpy to hydrate shadow buffers from the live Mach virtual memory
cache (bptr).
Under heavy memory pressure, this memcpy can trigger a Page Fault. If the
Mach kernel decides to evict dirty pages to satisfy the fault, the ext2fs pager
is invoked. The pager then attempts to write blocks to disk, which requires
acquiring the j_state_lock but that lock is already held by the suspended VFS
thread, causing a circular deadlock.
Fix:
Decouple memory hydration (memcpy) from the j_state_lock.
- While holding the lock, identify blocks needing hydration and stage them
in a thread-local list using jb_next.
- Temporarily increment t_updates to protect the transaction from being
freed.
- Drop the j_state_lock to perform the memcpy. If a page fault occurs here,
the pager can now safely acquire the lock to satisfy the I/O.
- Re-acquire the lock and decrement t_updates to finalize the commit.
This ensures the lock is never held while accessing pageable memory.
Test:
I have run the scenario in which i can recreate the deadlock multiple
times with debugging messages on and off, and i don't manage to cause this
specific
deadlock scenario any more.
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Summary of changes:
ext2fs/journal.c | 87 ++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 62 insertions(+), 25 deletions(-)
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